1. Field of the Invention
The invention relates to geophysics and notably to petroleum exploration using seismic methods.
In particular, the invention relates to a seismic data stratigraphic inversion method for obtaining images representative of a heterogeneous medium such as the subsoil to, for example, allow characterization of hydrocarbon reservoirs.
2. Description of the Prior Art
Seismic exploration methods produce disturbances in the subsoil (emission of seismic waves in the subsoil by a seismic source) and in observing at the surface the waves reflected on the interfaces between the various geological layers of the earth, referred to as formations (such as sandstone, sand, shale layers), or refracted along some of these interfaces. Each time the seismic waves encounter a geological interface, part of the waves are reflected and travel up to the surface where it can be detected and recorded by a seismic wave pickups. Particular devices allow multi-offset records of these waves to be obtained which is the principle of reflection surveys. The reflections linked with the emission of two types of waves are generally considered which are compression waves referred to as P waves and shear waves referred to as S waves.
The geological interfaces located during seismic surveys are the surfaces of separation of media of different elastic impedances. Elastic impedance generalizes the notion of acoustic impedance, acoustic impedance being by definition the product of the density of a rock by the rate of propagation of the P waves in the rock.
By nature, a seismic survey provides the position of the reflectors in two-way traveltime for reaching them and coming back. In order to know their real depth position (in meters for example), the rate of propagation of the waves has to be known to estimate time-depth conversion laws. Depth restitution allows direct comparison of the seismic traces. The definition of a reliable velocity field with existing techniques however remains a very delicate operation (especially in heterogeneous media) and record section interpretation is in most cases achieved in the time domain.
The main characteristics of reflected waves are linked, on the one hand, with the elastic impedance contrasts between the geological layers and, on the other hand, with the rates of propagation of the waves in the medium. Now, all these parameters depend on the nature and on the fluid filling degree of the subsoil. Producing a hydrocarbon reservoir therefore affects the propagation of the seismic waves in the rock. In the seismic records, this is translated into:
variations in the recorded amplitudes of the seismic waves that have travelled through the reservoir. These variations are linked with the elastic impedance changes in the reservoir;
time shifts in the recording of some seismic events associated with the same seismic reflector in depth also called misalignment of the events). These shifts can be more or less significant and they are linked with changes (or differences) in the propagation velocity field of the elastic waves being considered.
Recording reflected waves thus allows indirectly knowing the nature of the geological layers travelled through and possibly to monitor the evolution of their properties.
A known seismic data analysis method is stratigraphic inversion. Stratigraphic inversion is, in general terms, a technique allowing estimation of a model described by one or more parameters from indirect data. In general, this technique is used when it is not possible to directly measure the parameters. This technique implies that it is known how to solve the problem of predicting the data when the parameters of the model are known (modelling stage allowing obtaining of data referred to as synthetic data). This is for example the case within the context of petroleum exploration where geological and petrophysical data characterizing a three-dimensional reservoir are sought, but where only seismic data can be generally measured on a large scale. In this context, the goal of the invention is to determine parameters, such as the impedances relative to the P and S waves and/or the density, from seismic data obtained from waves emitted in the medium by a seismic source. These waves are propagated in the subsoil and reflected on the discontinuities of the medium. They are recorded by pickups coupled with the underground formation and collected by an acquisition device.
Advanced seismic exploration methods require analysis of several seismic data sets expressed on different time bases. These data are, for example:
data resulting from repeated seismic surveys, referred to as “4D seismic surveys”, carried out within the context of studies on the evolution of the distribution of fluids present in the subsoil, in a form of one or more reservoirs. These reservoirs can contain hydrocarbons, fluids of natural origin (natural gas, underwater, etc.), fluids deliberately injected into the ground (for CO2 or natural gas storage, or to improve the recovery rates in producing wells, etc.). The 4D seismic surveys are increasingly used in oil reservoirs after starting production. In this technology, (3D or 2D) seismic measurement is either repeated in the same place at different times or continuously measured by means of permanent pickups arranged at the surface or in wells. Comparison of the data recorded then allows monitoring the evolution over time of these reservoirs, generally hydrocarbon reservoirs, by mapping the movements of the fluids linked with production to optimize the development schemes for existing fields and new fields. Using repeated seismic measurements can also contribute to improving static subsoil models, notably in reservoir zones that are not affected by production thanks to the measurement redundancy;
multi-component seismic data acquired within the context of a seismic (2D or 3D) exploration survey. Joint interpretation of different types of measured seismic data allows improving characterization of the reservoirs or, more generally, of the subsoil. Multi-component seismic surveys involve measuring several types of waves reflected at the same time. It is in fact possible to measure, in addition to the PP waves corresponding to successive reflections of an incident P wave as a P wave, the PS waves corresponding to the incident P waves reflected as S waves. More generally, in the case of a seismic source that can generate S shear waves, it is also possible to record SS waves corresponding to successive reflections of an incident S wave as an S wave, and SP waves corresponding to the incident S waves reflected as P waves. In a case of high anisotropy due to the presence of faults in the reservoir or above it (referred to as overburden), splitting of the S waves into a fast S wave (along the fault lines) and a slow S wave (orthogonal to the fault lines) can be observed. Then SV and SH waves are discussed. Using the S waves can therefore be a good way to evaluate the relative anisotropy of the medium. Furthermore, the combination of P and S waves allows better detection of anomalies linked with the fluids because S waves are insensitive to the presence of fluids.
An important problem in the analysis of several data sets appears when these data sets are expressed in different time scales, which is the case with data obtained from repeated seismic surveys or multi-component seismic surveys. For example, within the context of multi-component seismic surveys (or surveys using converted waves), the problem is to be able to take into account the propagation time differences between the different types of waves reaching the receivers, many combinations being possible (PP, PS, SP, SS waves, etc.).
Thus, because of the time shifts (of 4D origin or linked with differences in the type of wave considered within the context of multi-component seismic surveys) that can complicate their interpretation, it is important to have specific tools allowing analysis of sets of seismic data expressed in different time scales.
In the sphere of 4D seismic surveys, a conventional method of analyzing 4D seismic records measures directly the amplitude differences between the seismic traces of the various available data sets. Interpretation is then often backed up by modelling the elastic behavior of the subsoil according to the assumed changes in the physical properties thereof. An example of this approach can be found in EP Patent Application 1,865,340, or in the following document:    Johnston, D. H., Eastwood, J. E., Shyeh, J. J., Vauthrin, R., Khan, M., and Stanley, L. R., 2000, “Using Legacy Seismic Data in an Integrated Time Lapse Study: Lena Field, Gulf of Mexico”, The Leading Edge, 19, no. 3, 294-302., p 294.
Interpretation of data based on amplitude differences can however be sometimes difficult. In fact, the variations over time of the physical properties of geological formations at the reservoir level (linked with the production of the reservoir, the use of enhanced recovery methods, etc.) modify the amplitude of seismic waves, but they also introduce time shifts in the seismic records (trace lengthening or shortening). The characteristic differences of two seismic traces can therefore be difficult to interpret since they result from amplitude changes as well as time shifts that can hide these amplitude variations.
Another approach uses statistical (or deterministic) pattern recognition techniques to split the seismic events into several groups, referred to as seismic facies, corresponding to the various lithologies and to the various physical states in the reservoir (construction of seismic facies maps). These classification type approaches are for example described in EP Patent 1,253,443 and in EP Patent Application 1,452,889. In some cases, the seismic records of each seismic survey are preprocessed via stratigraphic inversion, which allows construction of more readily interpretable seismic attributes such as the velocity of the impedance of elastic waves. These methods require using well data to establish statistical relations between the seismic attributes and characteristic parameters of the reservoir such as lithology, porosity, fluid filling degree, etc. The use of discriminant analysis techniques allows classification to be refined.
The drawback of these methods is that they are rather qualitative. They allow detection of the zones that have undergone changes, but quantitative estimation of the intensity of these changes remains difficult.
Other methods try to indirectly estimate the variations of physical properties in the reservoir in the form of seismic attribute variations obtained by means of seismic data stratigraphic inversion techniques. Stratigraphic inversion calculates seismic attribute models characteristic of the subsoil, such as the impedances and the density (generally in a reservoir-centered window) from the amplitudes of the seismic data, after or before stacking. By taking into account, in the inversion process, the available well data and physical constraints on the parameters that are sought (described as a priori information), stratigraphic inversion can provide reliable quantitative values for these seismic attributes, while improving the signal-to-noise ratio and the resolution of the models constructed.
A non-linear correlation algorithm for aligning two adjacent seismic traces is also known from Matson, Douglas G., and Hopper, J. R., 1992, “Nonlinear Seismic Trace Interpolation”, Geophysics, 57, no. 1, p. 136-145. It allows to determine a scale factor which thereafter allows re-interpolation of seismic traces while preserving the dip and amplitude changes of each seismic reflector, under certain hypotheses.
This method has been adapted for 4D records which is described as cross-correlation. It has a sliding window corresponding to input signal s1(t) by a quantity δt along the abscissa axis (time axis, t) and in seeking the time shift (δt) allowing the two signals to be superposed with a minimum difference. It is thus a criterion of a “resemblance” of the signals emitted and received, which, as can be sensed, gives results that are better since the distorsion is low.
To solve the problem of alignment of two seismic traces, there is another known technique based on a non-linear global optimization method coming from genetics. It is the Needleman-Wunsch (NW) algorithm, which is initially a method allowing alignment of amino-acid sequences in proteins.
Finally, there is a known method of analyzing several data sets by joint inversion of the seismic data sets, wherein a method of matching seismic data expressed in different time scales is used. Such a method is described in EP Patent 1,624,321 and in Agullo, Y., Macé, D., Labat, K., Tonellot, T., Bourgeois, A., and Lavielle, M., 2004, “Joint PP and PS Stratigraphic Inversion for Prestack Time Migrated Multicomponent Data”, 74th Annual International Meeting, SEG, Expanded Abstracts, 889-892. The chosen approach is based on the calculation of a scale factor with the sole objective of minimizing the dissimilarity between calculated impedance models each using separate types of data. In particular, this method hides the fact that this scale factor, depending on the ratio of the velocities of the P waves and the S waves, cannot have any value.
In general terms, the prior methods calculate scale factors that appear as free parameters and, in particular, do not respect the laws of physics. This can lead to physically impossible models.
The invention is a method of analyzing several seismic data sets expressed in different time scales, by joint stratigraphic inversion. The method comprises using a technique of matching the seismic events linked with the same interface or structure in depth, by a scale factor constrained by physical laws, so as to construct images representative of an underground reservoir which are physically plausible. The method thus allows reliable characterizing of the underground reservoirs.